Increasingly, data communications involve transmissions by optical sources that can deliver high volumes of digitized information as pulses of light. This is especially true for many communication companies that utilize laser diodes and optical fibers as their primary means for the transmission of voice, television and data signals for ground-based communications networks.
To achieve high bandwidth, laser diodes such as edge-emitting lasers and Vertical Cavity Surface Emitting Lasers (VCSELs) are commonly utilized as optical sources. These types of laser diodes are preferred due to their minute dimensions. For example, the typical VCSEL is measured in the order of micrometers. Consequently, an array of laser diodes can be integrated into a system to achieve high bandwidth transmissions.
In the manufacturing and production of VCSEL arrays, such as 1×12 or 1×4 parallel channel optical arrays, target optical and electrical characteristics are assigned to the arrays. To determine whether the VCSEL arrays will be operating at their target levels, each laser diode of the array is subjected to a burn-in process. That is, each VCSEL must be submitted to a quality control (QC) procedure that includes subjecting the VCSEL to a constant current at an elevated temperature for an extended time period. The burn-in current can be selected to be at a level that is higher than the standard operating current, since the QC procedure is a short-term test of whether the VCSELs will provide long-term performance during actual operating conditions. Similarly, the burn-in temperature is selected to be at a higher temperature than the anticipated operating temperature. Finally, the burn-in time period is selected on the basis of the type, specification and stringency of the devices.
Existing configurations use the laser driver to provide burn-in current to the laser diode. FIG. 1 is a block diagram of a conventional burn-in arrangement. Integrated circuit 10 includes an input buffer 12, limiting amplifier 14 and laser driver 16. An ASIC or other device provides signals to the integrated circuit 10 for applications such as communications. To perform the burn-in process, commands from an external digital controller 24 are submitted to an on-chip digital controller 22. The on-chip digital controller 22 sends commands to laser driver 16 that generates the high burn-in current for laser diode 20.
The use of an on-chip burn-in controller has several drawbacks. First, the on-chip digital controller 22 requires additional bonding pads on the integrated circuit 10. These extra pads add parasitics that degrade performance (e.g., eye quality), particularly for high transmission rates such as 2.5 Gbps or higher. Furthermore, bonding between the integrated circuit 10 and the digital controller 22 requires a delicate and difficult procedure.
Another drawback to the configuration of FIG. 1 is that the laser driver 16 provides the burn-in current. This may require redundant calibration circuitry used to control the accuracy of the laser driver 16 during the burn-in phase. Furthermore, using laser driver 16 as the burn-in current source may cause degradation of laser driver 16. The burn-in process is typically performed in a harsh environment (e.g., high temperature, high humidity), at a high DC current for a long time. This can result in degradation of laser driver 16.